In electronic braking systems for heavy goods vehicles, it is a legal requirement to provide failure redundancy in the brake system so that, in the event of a failure in the brake system, the driver is able to stop the vehicle in a reasonable distance. In an electronically controlled brake system for a heavy goods vehicle, this redundancy commonly takes the form of providing two pneumatic back-up circuits in addition to the normal electronic control to maintain braking capability in the event of an electrical power failure and/or the loss of a pneumatic reservoir. In order to achieve this, it is necessary to provide pneumatic connections between at least two separate pressurised fluid reservoirs and the arrangement of valves used to control supply of pressurised fluid to a brake actuator. It is advantageous to minimise the number of pipes used to provide the required back-up facility, particularly if the arrangement of valves is mounted adjacent the brake actuator in a wheel mounted unit, whilst the reservoir is mounted centrally on the chassis of the vehicle.